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Hardlink and symlink restrictions

A long-standing class of security issues is the link-based time-of-check-time-of-use race, most commonly seen in world-writable directories like /tmp. The common method of exploitation of this flaw is to cross privilege boundaries when following a given link (i.e. a root process follows a link belonging to another user). In Fedora 19, we permit links to only be followed when outside a sticky world-writable directory, or when the uid of the link and follower match, or when the directory owner matches the link's owner. In previous releases, this was enforced by SELinux policy and several services took advantage of private /tmp feature in systemd to eliminate or mitigate some potential vulnerabilities. In this release, we have enabled these restrictions via the following sysctl settings in /usr/lib/sysctl.d/00-system.conf as an additional layer of protection.